Gases that are to be shipped to various locations are generally packaged in portable vessels of various shapes and sizes which are capable of withstanding high pressures and which can be conveniently shipped. Typical of such vessels are the cylindrical containers commonly known as gas cylinders or bottles. These vessels are generally filled with gases by charging the gas into the vessel until the desired pressure is reached. The procedure is relatively simple and problem-free when the gas cylinder is to contain a single gas, or a mixture of gases of similar molecular weight where the concentration of each gas in the mixture is fairly high and not critical. However, when a gas container is to be filled to high pressure with a gas mixture comprising a large concentration of one component, for example concentrations of 75 volume % or more, and small quantities of one or more other components, for example concentrations of 10 volume % or less of each minor component, it is much more difficult to precisely measure the quantities of the low concentration gases.
A conventional procedure for filling gas cylinders with gas mixtures comprising a minor component and a major component is to first introduce the minor component into the cylinder using a low pressure gauge, and then introduce the major component into the cylinder to the desired end pressure using a high pressure gauge. Since precision pressure gauge readings are usually accurate to within about 0.1% of full scale, the error will be small when this procedure is used. Disadvantages of this method are that different gauges are required for measuring the components of the gas mixture and measurements of gas volumes based on pressures is not highly accurate due to the non-ideal nature of gas mixtures, particularly at high pressures. A further disadvantage arises if the minor compound is heavier than the major component. In this case, the first-filled minor component remains separated at the bottom of the gas cylinder for a prolonged period of time.
Methods and systems for accurately filling vessels with gas mixtures have been considerably investigated. U.S. Pat. No. 3,653,414 discloses a system and method for charging a thermostat with a mixture of a condensable medium and a noncondensable gas. The noncondensable gas is first introduced into the sensor of the thermostat to a predetermined pressure, measured by a first pressure gauge. A quantity of the condensable medium, measured by difference in pressure using a second pressure gauge, is then introduced into the sensor.
U.S. Pat. No. 3,669,134, discloses a gas measuring method in which two gases are charged into separate chambers using separate pressure regulators that are interconnected in such a manner that the pressures of the gases are in a predetermined ratio. The apparatus and method disclosed in this patent is complex and difficult to apply, particularly when it is desired to produce mixtures of three or more gases.
U.S. Pat. No. 4,219,038 discloses a gas mixing device for mixing a plurality of gases wherein each gas flows through a line that has a pressure regulator. In one embodiment of the disclosed invention the individual gases are stored in batteries of containers.
U.S. Pat. No. 4,688,946 discloses a method of mixing a liquid organic compound and a liquid propellant involving filling a metering cylinder with the liquid organic compound and then forcing the liquid organic compound, together with a predetermined volume of liquid propellant, into a mixing vessel.
U.S. Pat. No. 4,698,160 discloses apparatus for mixing two fluids for use in hemodialysis. Syringe type piston pumps are used to measure and force one or more of the components of the mixture into a mixing vessel.
U.S. Pat. No. 5,353,848 discloses procedure for accurately metering the components of a gas mixture into a gas cylinder while avoiding gas stratification, by introducing the gases into the cylinder in the order of their molecular weights using a differential pressure gauge.
U.S. Pat. No. 5,427,160 discloses a method of charging an oxidant gas and a flammable gas into a storage vessel wherein separate measuring chambers are used for each gas. The residual gas in the system lines is vented from the system.
Various efforts have been undertaken to effect a more rapid mixing of gas mixtures in cylinders. One technique is to introduce the lighter gas component into the bottom part of the cylinder by means of a mixing tube. This will force the lighter gas to migrate upwardly through the heavier gas, thereby causing the gases to mix. Another technique is to roll the cylinders until the contents are uniformly mixed. Each of these procedures requires considerable handling of the cylinders, which increases the cost of filling the cylinders with gas mixtures. A third method, described in U.S. Pat. No. 5,353,848, mentioned above, is to introduce first the lighter component and then the heavier component into the cylinder. In the last method mixing of the gases is accelerated due to the upward mobility of the lighter component and the downward mobility of the heavier component of the gas mixture.
Because of the importance of providing containerized gas mixtures in which the components of the mixtures are in precise composition, and the need to attain almost immediate homogeneity of vessel-contained gas mixtures, improved gas vessel filling methods are continuously sought. The present invention provides method and system which accomplishes these objectives.